This invention relates, in general, to semiconductor processing, and more particularly, to a method of reducing oxide precipitates in a superficial layer of a SIMOX structure.
Silicon On Insulator (SOI) structures are comprised of a substrate, an insulating layer on the substrate, and a superficial semiconductor layer on the insulating layer. One way of fabricating SOI structures is by using a Separation by Implantation of Oxygen (SIMOX) technique. In this technique the insulating layer or buried oxide layer is formed by implanting a high dose of oxygen into a semiconductor substrate. The implantation causes many defects, such as oxide precipitates, to form in the bottom of the superficial semiconductor layer of the SIMOX structure. Because active devices are built in the superficial semiconductor layer, a high defect density may prohibit the use of SIMOX structures for certain devices. The rough boundary between the superficial semiconductor layer and buried oxide caused by the presence of oxygen precipitate can cause adverse device characteristics. By decreasing the density and size of the defects, the interface between the buried oxide layer and the superficial silicon layer will also be made uniform. Thus it would be desirable to reduce the number and size of defects in the superficial layer.
A reduction in the defect density has been achieved by annealing the SIMOX wafer at a high temperature of approximately 1300.degree. to 1400.degree. C. after oxygen implantation rather than a conventional anneal of approximately 1200.degree.-1300.degree. C. Although annealing at high temperatures reduces the defect density somewhat, it would be desirable to reduce the defect density even further. In addition, it would be desirable to use a lower temperature anneal due to manufacturing constraints and contamination problems at such high temperatures. The high temperature is also limited to the melting point of silicon.
Another proposed method of reducing the defect density in SIMOX wafers is by implanting hydrogen ions into a semiconductor wafer after oxygen implantation and before anneal. This method has been shown to reduce the defect density in SIMOX wafers. However, it would also be desirable to reduce the defect density without having to employ an extra implantation process.
By now it should be appreciated that it would be advantageous to provide a method of reducing the density and size of oxide precipitates in a superficial layer of a SIMOX structure.
Accordingly, it is an object of the present invention to provide a method of reducing the density and size of oxide precipitates by utilizing a two-step anneal.
Another object of the present invention is to provide a method of reducing the density and size of oxide precipitates by using a low temperature anneal.
Yet another object of the present invention is to provide a method of reducing oxide precipitates and providing a pre-clean of the superficial semiconductor layer surface.